Audio-frequency amplifier



R. F. SHEA ETAL AUDIO FREQUENCY AMPLIFIER Jan. 2, 1945.

Fired May 1s, 1945 2 sheets-sheet 1 r( Inventors: Richard FT Shea,- CarPoH R. Miner Thehr` AttO-ney loa 400 FREQUENCIES AUDIO FREQUENCY AMPLIFIER Filed May 13, 1943 Fig. 5.

50 ZERO BIAS oN DEVICE I .02 VOLTS BETWEEN CONTACT I7 AND GROUND 2 Sheets-Sheet 2 -|00 DEVICES IANDa OPERATING NORMALLY WITH I5 voLTS BETWEEN CONTACT I7AND GROUND.

SAME BUT CATI-IODE OF DEVICE 2 COLD. `50- \G so IO'O 400 Io'oo Iqoo FREQUENCY Ioo- EFFECTIvENEss OF DEVICE "/a vs. VOLTAGE ON VOICE COIL Fi 4 7 T \,N C l2 6 I E :DI -IO- D '5 Ik 9 ,e L J Q u C' -B- BIAS on OEvIcez vs. vOIcE .4 a z COIL VOLTAGE C* Q l!) S s m D IIJ M- Iz -4 IOO CYCLE @AIN vs. BIAsvoLTAGE a E L! oF DEVICES IAND 2 IN PARALLEL n '2 l Q GAIN OF| DEVICES I-AND2IN PARALLEL 2v 40 @o so Ioo Igo- |40 |65 2go .2 .4 .e .s w La I4 Le 2.o

VOICE COIL vOLTs I Inventors: Ricnancl F. Shea, Car-Poll R. Miner;

Their` Attorney Patented Jan. 41945 UNITED STATES PATENT oFFicE l `Y `aaccscs AUDIO-FREQUENCYAMPLIFIEB manera F. shea, Fairfield, and cnn-u n. Miner,

Stratford, Conn., assignors to General Electric y f Company, a corporation of New York 4 Application May 13, 1943, Serial No. 486,778

(Cl. Z50-20) 8 Claims.

Qur invention relates to audio frequency ampliers and particularly radio receivers.

One of the objects of our invention is to provide such an amplifier having improved means for producing a frequency response characteristic so related to the threshold sensitivity characteristic of the human ear as to produce naturalness of tone, voice. or music, as reproduced by the `loudspeakerof the receiv As is well knownthe sensitivity of the human ear reduces at frequencies toward the lower limits of the voice or music range. Commonly, therefore, means are employed in radio receivers to increase the amplification of such low frequencies in order that the voice or music reproduced by the receiver sounds natural to'the human ear.

An object of our invention is to provide an improved amplification system providing such increase in ampliiication at low frequencies as to compensate for the reduced sensitivity of the human ear.

Another object of our invention is to effect this result with a reduction in the number of electron discharge devices employed and with simplification of the circuit structure.

The novel features which we believe to be characteristic. of our invention are set forth with particularity in the appended claims. Our invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference tothe following description taken in connection with the accompanying drawings in which Fig. 41 represents an embodiment of our invention; Fig. 2 represents certain output versus frequency characteristics thereof; Fig. 3 represents certain gain versus frequency characteristics thereof; and Fig. 4 represents certain other characteristics of an ampliiler employing our invention.

Referring to Fig. 1 of the drawings, `we have illustrated our invention as em'bodied in the audio frequency amplifler portion of a radio receiver, this portion comprising electrondischarge devices I, 2 and 3. One additional discharge device 4 is shown in the ligure and may represent the last radio frequency amplification stage of the receiver, as, for example, the last intermediate frequency amplier where the receiver is one of the superheterodyne type. The output circuit of this amplifier 4 comprises a tuned circuit 5 tuned to the intermediate frequency and which is coupled to an additional tuned circuit 6, tuned to the same frequency. This latter circuit 6 is conto ampliiiers, for use in A I is connected e resistance 24.

the input oi.' these two electron discharge amplil nectedbetween the cathode 'I of the amplifier I and an auxiliary cooperating anode, which may be incorporated in device I. vThe circuit between this anode and cathode includes the tuned circuit 6 and resistances 3 and I0. The resistance 5 'is the usual radio frequency iilter resistance. its upper terminal, as shown on the drawings, being connected to the cathode I through the cooperating nlter capacitor 9' to prevent radio frequency voltages appearing on load resistance IIl. The resistance I0 is the usual diode detector load resistance and is shunted by `a condenser I3, which is so proportioned relative to the resistance I0 that audio frequency voltages. which may represent voice or music with which the received carrier wave is modulated, appear across resistance III.

This audio frequency voltage is supplied through a switch Il and condenser I 5 to a volume control potentiometer resistance I6. A variable tap Il on the resistance I6 is connected through a coupling condenser I8 to the control electrode I9 of the ampliiier I and "also through a coupling condenser 2II to the control electrode 23 of the ampliiier 2. 'I'he cathode of the discharge device directly to ground and that of dis- 2 is connected to ground through which may be of 100 ohms. Thus charge device ners I and 2 are connected in parallel.

The anodes of these devices are connected together and through a resistance 25 to a source of operating potential the vpositive terminal of which may be represented by the conductor 26 and the negative terminal of which is grounded. Voltage may be supplied to this conductor 26 from any suitable source, such as a rectier of current of commercial household frequencies, for example, through the smoothing filter 21, 28 if desired. Audio frequency voltages which appear upon the resistance 25 are supplied through coupling condenser 3II and resistance 3I to the grid of the output amplifier 3 the anode circuit of which includes the primary winding of transformer 33. The secondary winding of this transformer 33 is connected speaker represented at tive force ampliiied in is reproduced as sound or music.

In the lower portion of the have represented a 35 whereby the electromothe amplifiers I, 2, and 3 waves representing voice drawings at II we phone jack phonograph pickup, which, when switch Il is in its left-hand posi- I tion. may supply electromotive force varying in accord with voice or music to the potentiometer the envelope of the discharge to the voice coil of a loud- I6, l1 ywhereby such electromotive force may be amplified by the ampliners l, 2 and 3 and-reproduced by the loudspeaker.

The discharge device l is connected to produce substantially uniform amplification at all frequencies. If desired, this discharge device may be one of the SSQTtype.

Thel discharge device 2 is one having a cathode 42 connected to ground through the resistance 24 and having, in addition to the control electrode 23, screen electrodes 43 and a suppressor elec- .trode 44, the latter being connected directly to thecathode'. This device may, desirably, be of the 68H7 type and has extremely high Gm, high amplification and an anode current-grid voltage characteristic having a sharp lower knee, or cutoil, at low grid voltages. This device thus has higher ampliflcation'than does the GSQ'l.

In accordance with our invention this device is connected to regenerate at low frequencies and to degenerate at high crease its amplification at low frequencies and to decrease its amplification at high frequencies.'

For this purpose voltage is supplied from the upper terminal of the secondary winding of transformer 33 through condenser` 34 to the screen grids 43 of device 2 in degenerative phase at all frequencies above about one hundred cycles, this feedback being regenerative at lower frequencies due to phase shift in transformer 33 and condenser 34. Condenser 34 may have a capacity of about one-tenth of a microfarad. Voltage is also supplied from the upper terminal of the primary windingof transformer 33 through resistance 45 and condenser 46 to the cathode 42 of thedischarge device 2. Higli-l frequencies, as

those above three thousand cycles, however, are

bypassed from this circuit by condenser 41. Lower frequencies, but those above 150 cycles, are supplied through this circuit to the -cathode of the device 2 in degenerative phase so as to reduce the amplication ofthis device. At frequencies still lower, however, as in the neighborhood of 100 cycles, the phase shift produced by the condenser 46, which may be of the order of onehundredth of a microfarad, and resistance 24,

which may be of the order of 100,000 ohms, is such that discharge device 2 becomes regenerative, producing a rise in amplication in this device at a frequency somewhat below 100 cycles.

Thus both of the two feedback paths, one to the screen grid 43 and the other to the cathode resistor 24, work to reduce amplification by degeneration at frequencies above the neighborhood of 100 cycles and 'both become regenerative at lower frequencies to produce a sharp regenerative peak just below 100 cycles. Either of, these two feedback paths may be omitted but both are desirable to secure the required accentuation of low tones at low volume.

A very considerable accentuation may be obtained by proportioning e'ther or both of these circuits to reduce degenerative at low frequencies even though they do not become regenerative at any frequency in the audio range'. However, while these circuits may be proportioned-to produce the degree of accentuation required in any particular case, to secure extreme accentuation it is desirable that both of these circuits become regenerative at a very low frequency.

Means responsive to the volume of the amplied signal is provided for control of the amplification of the discharge `device 2. prises the circuit comprising condenser 50 and This comfrequencies thereby 'to inresistance 5| by which voltage is supplied fromi the anodeof the output discharge device 2 to the anode 52, which cooperates with the cathode in the discharge device 4 as a diode rectier producing unidirectional voltage across' the diode load resistance.. This diode' load resistance comprises resistances`54land'24. The unidirectional voltage on' resistances I4 and 24 is supplied through resistances 55 and 53 to the control elec,- trode 23 of thedevice 2. Condenser 5I! is of such a value that substantially all currents in the voice range are supplied to the anode 52 to 'pro--v ,duce rectified voltage on the resistances 54 and 24 thereby to reduce the amplification of the device 2.

Thus in the operation of the system as the volume increases, the amplication of the amplifier 2. is gradually reduced so that the overall frequency characteristic; of the system becomes more nearly the characteristic of the amplier l which is substantially fiat. At low volumes, however, the amplifier 2 is effective to amplify the low frequencies to an extent much greater than the high ffrequencies thereby to produce the required accentuation of the low frequencies relative to the high frequencies.

'I'his operation is illustrated by the characterfrequency 'when the outputy in the loudspeaker at 400 cycles is, respectively, .01, .05, .1, .25 and .5 watt. It will be' seen that the output at frequencies below cycles is very substantially greater in relation to the'output at 400 cycles when the output from the loudspeaker is .01 watt, as represente@ by the curve A, than when it is .5 watt, as represented by the curve E.

In Fig. 3 are shown a number of characteristics representing the relation betweenfrequency plotted as abscissas and gain plotted as ordinates. In this figure the curve F represents the gain of the system when the tap l1 is adjusted to supply .13 volt to the control electrodes of devices I and 2. This curve corresponds to the conditionin which discharge devices l and 2 are operating normally, and producing fairly loud volume from the loudspeaker. The curve G represents this same condition but with the cathode of discharge device 2 deenergized so that this device is in-` active. It will be seen that these curves are sublstantially identical at all frequencies above about 300 cycles but that the curve F rises increasingly above the curve G at the lower frequencies. This difference in the two curves is due to the operation of the discharge device 2.

The curve I represents a condition of very low volume. This curve illustrates the operation of the system at different frequencies when the voltage between contact l1 and ground is l.02 volt with ashort circuit connected across resistance 54 thereby to remove the volume control effect of diode 52. Thus this curve represents the extreme gain at very low frequencies obtainable by action of the system as described if the automatic volume control action of diode 52 did not occur.

This very high gain at low frequencies and at low volume is highly desirable. However, as the volume rises, if nothing further were done, this high gain would result in undesired accentuation of the low tones andgin distortion of the low tones due to overloading of device 3 and the i'iow of arid current in that device.

These undesired eil'ects are avoided in the sys- `might be represented by 'a curve such as that represented at H lying somewhere between curves I and F at a position dependent upon the volume. In Fig. 4 the curves J `and K represent the relation between the bias produced on the control electrode of the discharge device 2 plottedeas ordinates at the left-hand side of the graph and voltage on the voice coil plotted as abscissa, the curve J corresponding to 100 cycles and the curve K corresponding to 400 cycles. It will be seen thatthese curves are linear tical purposes. of the same pitc The curve L represents the gain at 100 cycles of the two tubes in parallel plotted as abscissa versus the bias on discharge device 2 plotted as ordinate at the left-hand side of the curve. It will be seen that the gain increases very greatly as the bias is reduced and that the gain has a minimum at about 12 volts bias, this minimum being represented by the vertical line M. This minimum gainof device 2 is about equal to the gain of device I. All of the gain represented by the disaccesos Letters Patent of the United States, is:

and, for all pracrangement and in the intrumentalities employed may be'made, and we contemplate bythe appended claims to cover any such modifications as fallwithin 4the true spirit and scope of our invention. v

' What we claim as new and desire to secure by 1. AIn combination, `a pair of amplifiers, each of said amplifiers having an anode, acathode and a control electrode, means to supply signal electromotive force having frequencies extending over a wide range and having substantially like frequency characteristics between the cathode and-- control electrode of each of said amplifiers in parallel, a common output circuit connected between 'said anode and cathode of both of said amplifiers, one of said amplifiers having substan- 'tially uniform amplification at all frequencies to be amplified. means to render the other of said amplifiers regenerative at low frequencies and degenerative at high frequencies in said range, and means responsive to the intensity of said signal electromotive force to reduce the ampliilcation of said other amplifier as said intensity increases, whereby said signal electromotive force is reproduced in said common circuit with low frequency components accentuated at low volume.

tance between4 the lines M and L is, of course,

the increased gain` at low volume produced by vthe discharge device 2.

Curve N represents the relation between the voltage on the voice coil plotted as abscissa, and the decibels change in outputproduced by deenergizing the cathode of discharge device 2 plotted as ordinate at the right-hand' side of the graph. 'I'hiseurve shows the extreme eiectiveness of the discharge device 2 at low volumes and its reduced effectiveness at high volumes.

' In the operation of the system as thus described it is found that on large volume the amplification of the discharge device 2 is not reduced to zero but is reduced to a value about equal to the amplification of discharge device I. If desired, 'discharge device l removed from the circuit although the range of accentuation of low tones at low volume will be reduced. A diode lwould have to be substituted as-a detector replacing the diode comprising the anode l and the cathode 1.

It will be observed that in the system as thus described the control electrodes of both of discharge devices I and 2` are connected directly to the volume control potentiometer Il. the neces- -sity for the interposition of any amplification stage between the volume control and the devices I and 2 beingentirely obviated by our inplied to both control electrodes with like frequency characteristics.

, While we have shown a particular embodiment of vour invention, it -Will of course be understood that we do not wish to be limited thereto since Adifferent `modifications both in the circuit arl may be entirely 2. In combination, a pair of amplifiers, each of saidfampliflers having an anode, a cathode and a control electrode, means to supply signal electromotive force having frequencies extending over a wide range and having substantially like frequency characteristics between the cathode and control electrode of each of said amplifiers in parallel, a common output circuit connected between said anode and cathode of both of said ampliers, one of said ampliers having substantially greater amplication than the other, means responsive to increase in intensity of said signal electromotive force to render said control electrode of said one 'amplifier increasingly negal ner,

tive with respect to its associated cathode,` and meansto supplysignal electromotive fforce from the anode of said other amplifier between the control .electrode and cathode'thereof in phase to increase the gain of said amplier at low frequencies, whereby low frequency components of signal electromotive force in said common circuit are accentuated with respect to high frequency components to an extent tromotive force is of low high intensity.

3.- 'Ihe combination, in a voice .current ampliof means to render said amplifier regenerative at low frequencies in degenerative at high frequencies in said range, andl means responsive to currents of all frequencies amplified the gain of said amplier at all frequencies, whereby low frequency currents are increasingly accentuated with respect to high frequency currents by said amplifier as the intensity'of currents amplified by said amplifier is reduced.

4. Incombination, a pair of `electron discharge intensity than when of amplifiers connected in cascade, each of saidv ampliners having an anode, a cathode and a congenerative at frequencies low in the voice range and degenerative at frequencies high in said voice range, said regeneration being to such a degree that at minimum volume level a sharp rise in total amplification with reduction in frequency greater when said electhe voice range and by said amplifier to reduce occurs in'the region of 100 cycles, said rise being to such an exitent that upon slight rise from minimum intensities the control electrode of said4 last amplifier becomes positive with respect to the cathode thereof producing undesired rectiflcation, and means responsive to all voice or music components of voltage present in the output of the last of said amplifiers to reduce the gain of said first amplifier at all frequencies to an extent suillcient-to prevent said rectification between the control electrode and cat ode of said last ampliiler.

5. The combination, in a radio receiver for l waves modulatedin accord with voice or music,

of a detector for said waves having an output circuit including a volume control potentiometer resistance, a pair lof electron discharge ampliers having a common output circuit, means to supply voltage existing between one terminal of said resistance and a point variable along said resistance to both of said amplifiers with like frequency characteristics, one of said ampliers having higher 'Gm than the other, means responsive to increase in voltage in said common circuit to decrease the gain of said one ampliner, and means to render said. one amplifier regenerative at frequencies below one hundred cycles and degenerative at higher frequencies whereby low frequency currents in said common circuit are accentuated with respect to high frequency currents at low volume and said accentuation is reduced at high volume.`

6. The combination, in a radio receiver, of a pair of electron discharge ampliers connected in parallel, means to supply audio frequency voltages to the input to both of said amplifiers for amplification thereby, means to supply amplified voltage'from the output of said amplifiers to the input of one of said amplifiers in degenerative phase at high audio frequencies to reduce ampliflcation at said frequencies, means to shift the phase of said voltage supplied from said output to said input as the frequencies approach the lowest frequency ln the range to be amplified* thereby to increase amplification at low fre- 4 l aseases n quency, and means to reduce rthe gain of said one ampliner in response to increase in intensity of the currents amplified by said amplifiers.

7. The combination, in a radio receiver, of a pair of electron discharge ampliiiers connected in parallel, one of said ampliers having a screen electrode, cathode and control electrode, and said ampliflers having a common audio frequency input circuit and a common output circuit, means to supply from said common output circuit voltages between said cathode. and said screen elec-ftrode and between 'said control electrode and cathode both in degenerative phase at high audio frequency, said means including means to shift.

the phase of both of said voltages to reduce degeneration at the lowest frequenciesto be amplifled thereby to accentuate the low tones, and means to reduce the gain of said one amplifier as the intensity of currents amplified increases thereby to reduce said accentuation of low tones at high volume.

8. The combination, in a radio receiver. of a detector, a potentiometer in the output of said detector on which voltage varying in accord with voice and music appears, a pair of ,electron discharge amplifiers, each having .input electrodes. means to supply voltage from said potentiometer to the input electrodes of both amplifiers with substantially like frequency characteristics, a common output circuit for said amplifiers, means to supply voltage from said common output circuit to the input of one of said amplifiers in degenerative phase at high audio frequencies,'said means including means to shift the phase of said voltage to reduce said degeneration sufficiently materially to increase amplification of the lowest 

